1. Field of the invention
This invention relates to a toner for developing electrostatic latent images, which is used to visualize electrostatic latent images, and an image-forming method making use of the toner.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691 are known as electrophotography. In general, using a photosensitive member comprising a photoconductive material, copies or prints are obtained by forming an electrostatic latent image on the photosensitive member, subsequently developing the latent image by the use of a toner to form a visible image (toner image), transferring the toner image to a transfer medium (a recording medium) such as paper, and thereafter fixing the toner image onto the transfer medium by the action of heat and/or pressure.
Various methods have been proposed as methods for fixing the toner image. For example, what is widely used is a method of fixing the toner image while holding and transporting a transfer medium (such as paper) having an unfixed toner image on its surface, between a heat roller kept at a stated temperature and a pressure roller having an elastic layer and coming into pressure contact with the heat roller. In this method, however, the toner image comes into contact with the heat roller surface in a molten state under application of pressure, and hence part of the toner image may adhere and transfer to the surface of a fixing roller, tending to cause what is called an offset phenomenon, a phenomenon in which the toner having adhered to the fixing roller surface is again transferred to the next transfer medium.
Especially when images are formed using full-color toners, the offset phenomenon is liable to occur if a heat history at too high temperature is given in order to effect color formation of what is called the secondary color formed by color mixing of monochromatic toners superimposed in multiple layers and thereafter melted by heating.
In order to prevent toner from adhering to the fixing roller surface, a measure has been hitherto taken such that the roller surface is formed of a material having an excellent releasability for toner (e.g., silicone rubber or fluororesin) and, in order to prevent offset and to prevent fatigue of the roller surface, its surface is further covered with a thin film formed using a fluid having a good releasability as exemplified by silicone oil. However, this method, though effective for the prevention of the offset of toner, requires a device for feeding an anti-offset fluid, and hence has such a problem that the fixing assembly must be made complicated. Thus, it is not a preferable direction to prevent the offset by feeding the anti-offset fluid. Rather than such a measure, under existing circumstances, it is sought to provide a toner having a broad low-temperature fixing range and high anti-offset properties.
Accordingly, in order to improve the release properties of the toner, it has been put into practice to add a wax such as low-molecular-weight polyethylene or low-molecular-weight polypropylene that may well melt at the time of heating. The use of wax is effective for preventing offset, but on the other hand results in an increase in agglomerating properties of toner and tends to make charging performance unstable and cause a lowering of running performance. Accordingly, as other methods, it is variously attempted to improve binder resins.
For example, a method is known in which the glass transition temperature (Tg) and molecular weight of a binder resin in toner are made higher to improve the melt viscoelasticity of the toner. Such a method, however, causes such a problem that the improvement in anti-offset properties may result in an insufficient fixing performance to deteriorate fixing performance in low-temperature fixing, i.e., low-temperature fixing performance, which is required for the achievement of high-speed copying and energy saving.
From the above viewpoint, in order to improve the low-temperature fixing performance of toners, it is necessary to decrease the viscosity of toner at the time of its melting and increase the contact area with a fixing substrate. For this reason, it is required to lower the Tg and molecular weight of binder resins to be used.
The low-temperature fixing performance and the anti-offset properties conflict with each other in some phase, and hence it is very difficult to provide toners satisfying these performances at the same time.
To solve this problem, for example, a toner comprising a vinyl polymer cross-linked to an appropriate degree by adding a cross-linking agent and a molecular-weight modifier is proposed, as disclosed in Japanese Patent Publication No. 51-23354. In Japanese Patent Publication No. 55-6895, a toner is proposed which has as a constituent unit an .alpha.,.beta.-unsaturated ethylene monomer and has a broad molecular-weight distribution so as for the ratio of a weight-average molecular weight to a number-average molecular weight (Mw/Mn) to be 3.5 to 4.0. A toner having a blend type resin comprising a vinyl polymer whose Tg, molecular weight and gel content are specified is also proposed.
The toners according to these proposals certainly have a broader fixing temperature range between the lowest fixing temperature (the lowest temperature at which the fixing is possible) and the offset temperature (the temperature at which the offset begins to occur). There, however, has been such a problem that it is difficult to make their fixing temperature sufficiently low when a satisfactory anti-offset performance is imparted to the toner and on the other hand the anti-offset performance comes to be insufficient when importance is attached to the low-temperature fixing performance.
For example, Japanese Patent Application Laid-open No. 56-158340 discloses a toner having a binder resin comprised of a low-molecular-weight polymer and a high-molecular-weight polymer. In practice, it is difficult for this binder resin to be incorporated with a cross-linking component. Hence, in order to improve anti-offset properties, it is necessary to make the high-molecular-weight polymer have a large molecular weight or to increase the proportion of the high-molecular-weight polymer. This takes a course toward a great lowering of pulverizability of resin compositions, and makes it hard to obtain satisfactory results in practical use.
With regard to a toner comprising a blend of a low-molecular-weight polymer and a cross-linked polymer, Japanese Patent Application Laid-open No. 58-86558 discloses a toner having as main resin components a low-molecular-weight polymer and an insoluble infusible high-molecular-weight polymer. According to such a means, the fixing performance of toners and the pulverizability of resin compositions are considered to be improved. However, the low-molecular-weight polymer has a ratio of weight-average molecular weight/number-average molecular weight (Mw/Mn) as small as 3.5 or less and the insoluble infusible high-molecular-weight polymer is in a proportion as large as 40 to 90% by weight, and hence it is difficult to satisfy both of the anti-offset properties of toners and the pulverizability of resin compositions. In fact, it is very difficult to produce toners well satisfying the fixing performance and the anti-offset properties, unless a fixing assembly having a system for feeding the anti-offset fluid is used.
Moreover, the use of the insoluble infusible high-molecular-weight polymer in a large quantity may result in a very high melt viscosity when materials are heat-kneaded in the production of toner, and hence the materials must be heat-kneaded at a temperature much higher than usual instances, so that additives undergo thermal decomposition to make the toner have low performances. The above toner has such a problem.
Japanese Patent Application Laid-open No. 56-16144 discloses a toner containing a binder resin component having, in its molecular-weight distribution as measured by GPC (gel permeation chromatography), at least one peak value in each of the regions of a molecular weight of from 10.sup.3 to 8.times.10.sup.4 and a molecular weight of from 10.sup.5 to 2.times.10.sup.6. In this instance, the binder resin component can have a superior pulverizability, and the toner is superior in anti-offset properties and fixing performance, can be well prevented from causing its filming or melt-adhesion to photosensitive members and can have a superior developing performance. However, it is required to more improve the anti-offset properties and fixing performance of the toner. In particular, it is difficult for this resin to cope with nowaday's severe demands while more improving the fixing performance and keeping or improving other various performances.
Thus, it is very difficult to achieve the performances concerning the fixing of toners (i.e., low-temperature fixing performance and anti-offset properties) at a high level.
As means for preventing the offset phenomenon, for example, Japanese Patent Applications Laid-Open Nos. 1-214872, 2-204752, 2-204723, 3-77962, 3-284867 and 4-81863 also disclose toners containing a binder resin and a wax, having superior fixing performance and anti-offset properties. For example, Japanese Patent Application Laid-Open No. 5-6029 discloses a toner having, in its molecular-weight distribution as measured by GPC, less than 15% of a molecular-weight region of 5,000 or less, not less than 5% of a molecular-weight region of 5,000,000 or more, and a main peak in the region of molecular weight of from 5,000 to 100,000, and having a weight average molecular weight of 5,000,000 or more. In this instance, the toner can have superior low-temperature fixing performance and anti-offset properties, be well prevented from causing its filming or melt-adhesion to photosensitive members and have a superior developing performance.
In the categorization of toner production processes, the above means of preventing offset is achieved by the pulverization process, i.e., a process in which a colorant comprising a dye or pigment is melt-kneaded with a thermosetting resin so as to be uniformly dispersed therein, the product obtained is thereafter pulverized by means of a fine grinding mill, and the pulverized product obtained is classified by means of a classifier so as to have the desired toner particle diameters. However, the anti-offset properties can be more effectively improved even by the suspension polymerization process, i.e., a process in which a polymerizable monomer, a colorant and a polymerization initiator, further optionally together with a cross-linking agent, a charge control agent and other additives are uniformly dissolved or dispersed to prepare a monomer composition, and the monomer composition is dispersed in a continuous phase containing a dispersion stabilizer, e.g., in an aqueous phase, by means of a suitable agitator to carry out polymerization reaction so as to have the desired toner particle diameters. For example, Japanese Patent Application Laid-open No. 5-88409 discloses a toner having what is called the core/shell structure wherein a low-softening substance is covered with a shell resin, which is obtained by uniformly dissolving or dispersing the low-softening substance in the monomer composition, also setting the polarity of the low-softening substance in the monomer to be smaller than that of the main monomer, and still also adding in a small quantity a resin or monomer having a great polarity. In this instance, a toner that may hardly cause the filming onto photosensitive members or any contamination of the surfaces of toner carrying members (developing sleeves) and has superior running performance and developing performance, can be obtained without damaging the low-temperature fixing performance.
However, recent copying machines and printers are strongly demanded to be made small-sized, light-weight and highly reliable, and toners are also severely demanded to have higher performances. For example, it is sought to provide a toner with superior performances that may more hardly cause the filming onto photosensitive members or any contamination of the surfaces of toner carrying materials or members such as carriers and sleeves and has superior running performance and developing performance, without damaging the low-temperature fixing performance.
Japanese Patent Applications Laid-Open Nos. 59-21845, 59-218460, 59-219755, 60-28665, 60-31147, 60-45259, 60-45260 and 3-197971 disclose toners having a superior fixing performance, in which insoluble matters of toners, which are insoluble in solvents such as THF (tetrahydrofuran) and toluene, are specified. Under existing circumstances, however, these are sought to be more improved from the point of view of the achievement of both of the low-temperature fixing performance and the running performance.
Japanese Patent Applications Laid-Open Nos. 60-31147 and 3-197971 discloses toners in which the molecular weights of their soluble matters are also specified. Under existing circumstances, however, these are sought to be more improved in the running performance.
Japanese Patent Application Laid-Open No. 3-251853 discloses a toner obtained by suspension polymerization, the toner having a plurality of peaks in its molecular-weight distribution, where the peak of the smallest molecular weight is located at 50,000 or less and the peak of the largest molecular weight is located at 200,000 or more. Under existing circumstances, however, this is sought to be more improved in the low-temperature fixing performance.
Japanese Patent Application Laid-Open No. 3-39971 discloses a color toner having, in its molecular-weight distribution as measured by GPC, a peak Mp1 in the region of molecular weight of from 500 to 2,000, a peak Mp2 in the region of molecular weight of from 10,000 to 100,000, and having a weight-average molecular weight (Mw) of from 10,000 to 80,000 a number-average molecular weight (Mn) of from 1,500 to 8,000 and a ratio of Mw/Mn of not less than 3 can obtained. In this instance, a color toner that has superior anti-offset properties and can form sharp color images with a high chroma can be obtained. However, it has become necessary to provide a toner that may more hardly cause the filming onto photosensitive members or any contamination of the surfaces of toner carrying materials or members such as carriers and sleeves.
Meanwhile, in conventional electrophotographic processes, toner particles not transferred to the transfer medium after the transfer step and having remained on the surface of a photosensitive member are commonly removed from the surface of the photosensitive member through a cleaning step making use of a cleaning means. Blade cleaning, fur brush cleaning or roller cleaning is used as the cleaning means. From the viewpoint of apparatus, the whole image-forming apparatus must be made larger in order for the apparatus to have the cleaning means. This has been a bottleneck in attempts to make apparatus compact.
From the viewpoint of ecology, a cleanerless system or toner reuse system that may produce no waste toner is long-awaited in the sense of effective utilization of toners.
For example, Japanese Patent Publication No. 5-69427 discloses a technique called "cleaning-at-development" (cleaning simultaneously performed at the time of development) or "cleanerless" system. In such a method, one image is formed at one rotation of the photosensitive member so that any effect of transfer residual toner does not appear on the same image. Japanese Patent Applications Laid-Open Nos. 64-20587, 2-259784, 4-50886 and 5-165378 disclose methods in which the transfer residual toner is dispersed or driven off by a drive-off member to make it into non-patterns so that it may hardly appear on images even when the surface of the same photosensitive member is utilized several times for one image. There, however, has been a problem of image deterioration. Japanese Patent Application Laid-Open No. 5-2287 discloses a constitution in which the toner charge quantity around the photosensitive member is specified so that any positive memory or negative memory caused by the transfer residual toner may not appear on images. It, however, does not disclose any specific constitution as to how to control the toner charge quantity.
In Japanese Patent Applications Laid-Open Nos. 59-133573, 62-203182, 63-133179, 2-302772, 4-155361, 5-2289, 5-53482 and 5-61383, which disclose techniques relating to the cleanerless system, it is proposed, in relation to imaging exposure, to make exposure using light having a high intensity or to use a toner capable of transmitting light having an exposure wavelength. However, only making exposure intensity higher may bring about a blur in dot formation of a latent image itself to cause an insufficient isolated-dot reproducibility, resulting in images having a poor resolution in respect of image quality, in particular, images lacking in gradation in graphic images.
As for the means making use of the toner capable of transmitting light having an exposure wavelength, the transmission of light certainly has a great influence on the fixed toner having been made smooth to have no particle-particle boundaries, but, as mechanisms of screening exposure light, it has less influence because it more chiefly concerns the scattering of light on the toner particle surfaces than the coloring of toner itself. Moreover, colorants of toners must be selected in a narrower range, and besides, at least three types of exposure means having different wavelengths are required when full-color formation is intended. This goes against making apparatus simple, which is one of the features of the cleaning-at-development.
Contact charging carried out by bringing a charging member into contact with the photosensitive member and contact transfer carried out by bringing a transfer member into contact with the photosensitive member interposing a transfer medium between them may commonly generate less ozone and is a system preferable from the viewpoint of ecology. The transfer member serves also as a transport member for transfer mediums, and the system has such a feature that the apparatus can be easily made compact. If, however, the cleaning is not sufficient at the developing zone, the charging member and the transfer member are liable to be contaminated, tending to cause image stain, transfer medium back stain, or blank areas caused by poor transfer (middle portions of line areas are not transferred), due to poor charging of the photosensitive member, and this further accelerates image deterioration. There have been such problems.
In addition, in the cleaning-at-development, in which no cleaning assembly is substantially provided, it is essential for the system to be so set up that the surface of a latent image bearing member is rubbed with the toner and a toner carrying member. This may cause toner deterioration, deterioration of the toner carrying member surface and deterioration or wear of the latent image bearing member surface as a result of long-term service, which leave a problem of deterioration of running performance that has not been well solved in the prior art, and it has been sought to bring out a technique for improving the running performance.
In particular, it has been considered necessary to better prevent the latent image bearing member surface, i.e., the photosensitive member surface, from contamination with toner. In the past, to solve such a problem, it has been proposed to impart releasability or lubricity to the toner or photosensitive member. For example, Japanese Patent Publication No. 57-13868, Japanese Patent Applications Laid-Open Nos. 54-58245, 59-197048, 2-3073 and 3-63660 and U.S. Pat. No. 4,517,272 disclose a method in which a silicone compound is incorporated in the toner. Japanese Patent Application Laid-Open No. 56-99345 discloses a method in which a lubricating substance as typified by a fluorine-containing compound is incorporated in the surface layer of a photosensitive member.
However, there is no example where these methods are applied in the system called cleanerless or cleaning-at-development, having substantially no cleaning assembly.
In recent years, various organic photoconductive materials have been brought out as photoconductive materials of electrophotographic photosensitive members. In particular, photosensitive members of a function-separated type in which a charge generation layer and a charge transport layer are formed in superposition have been put into practical use, and are mounted on image-forming apparatus such as copying machines, printers and facsimile machines. As charging means in such electrophotography, means utilizing corona discharging have been used. Since, however, the use of corona discharging generates ozone in a large quantity, the appratus must have a filter, and there have been such a problem that the apparatus must be made large in size and the running cost increases.
As techniques for solving such problems, charging methods have been proposed in which a charging member such as a roller or a blade is brought into contact with the surface of the photosensitive member so as to form a narrow space in the vicinity of the contact portion, and the discharge as can be explained by what is called the Paschen's law is formed so that the generation of ozone can be prevented as much as possible. In particular, a roller charging system making use of a charging roller as the charging member is preferably used in view of the stability of charging.
Specifically, in the roller charging system, the charging is carried out by discharge from the charging member to the member to be charged, and hence the charging takes place upon application of a voltage above a certain threshold value. For example, when a charging roller is brought into pressure contact with an OPC (organic photoconductor) photosensitive member with a 25 .mu.m thick photosensitive layer, the surface potential of the photosensitive member begins to rise upon application of a voltage of about 640 kV or above, and at voltages above a threshold value the photosensitive member surface potential linearly increases at a slope of 1 with respect to the applied voltage. This threshold value voltage is hereinafter defined as charging starting voltage Vth. Namely, in order to obtain a photosensitive member surface potential Vd, a DC voltage of Vd+Vth which is higher than necessary must be applied to the charging roller. However, the resistivity of the contact charging member varies depending on environmental variations, and hence it has been difficult to control the potential of the photosensitive member at the desired value.
Thus, in order to achieve more uniform charging, as disclosed in Japanese Patent Application Laid-open No. 63-149669, AC charging is used which is a method of applying to the contact charging member a voltage produced by superimposing an AC component having a peak-to-peak voltage of 2.times.Vth or above, on a DC voltage corresponding to the desired Vd. This method aims at a potential-leveling effect which is attributable to AC, where the potential of the member to be charged converges on Vd, the middle of a peak of AC potential, and may hardly be affected by external disturbance such as environmental variations.
However, even in such contact charging assemblies, their fundamental charging mechanism utilizes the phenomenon of discharging from the charging member to the photosensitive member. Hence, as previously stated, the voltage necessary for charging must be at a value beyond the surface potential of the photosensitive member. When AC charging is carried out for the purpose of achieving uniform charging, the electric field of AC voltage may remarkably cause vibration and noise of the charging member and photosensitive member, and the discharge may remarkably cause deterioration of the surface of the photosensitive member. This involves another problem.
Japanese Patent Application Laid-Open No. 61-57958 discloses an image-forming method in which a photosensitive member having a conductive protective film is charged using conductive fine particles. This publication discloses that a photosensitive member having a semiconductive protective film having a resistivity of from 10.sup.7 to 10.sup.13 .OMEGA..cm is used as the photosensitive member and this photosensitive member is charged using conductive fine particles having a resistivity of 10.sup.10 .OMEGA..cm or below, whereby the photosensitive member can be evenly and uniformly charged by discharging, without injection of charges into the photosensitive layer, and good images can be reproduced. According to this method, the vibration and noise in the AC charging can be prevented. However, since the photosensitive member is charged by discharging, the deterioration of the photosensitive member surface, caused by the discharging, may still occur, and also it has been necessary to use a high-voltage power source. Hence, it has been sought to carry out charging by direct injection of charges into the photosensitive member.
Japan Hardcopy '92 Papers, p.287, "Contact Charging Performance Using Conductive Roller", discloses a method in which a voltage is applied to a contact charging member such as a charging roller, a charging brush or a charging magnetic brush, and charges are injected into trap levels present at the photosensitive member surface to carry out contact injection charging. This method is a method in which charges are injected into a dark-portion insulating photosensitive member by means of a low-resistivity charging member to which a voltage has been applied, and has been conditioned on a sufficiently low resistivity of the charging member and also on its surface to which a material (such as conductive filler) providing the charging member with conductivity is sufficiently laid bare.
Hence, it is reported also in the above publication that aluminum foil or an ion-conductive charging member made to have a sufficiently low resistivity in an environment of high humidity is preferable as the charging member. Studies made by the present inventors have revealed that the resistivity of charging members at which charges can be sufficiently injected into photosensitive members is 1.times.10.sup.3 .OMEGA..cm or below and, at a resistivity higher than that, a difference begins to occur between applied voltage and charge potential to cause problems on the convergence of charge potential.
However, when the charging member having such a low resistivity is actually used, excess leak currents may flow from the contact charging member to scratches and pinholes produced on the photosensitive member surface to tend to cause faulty charging around them, expansion of the pinholes and electrification failure of the charging member.
To prevent such problems, it is necessary to make the charging member have a resistivity of about 1.times.10.sup.4 .OMEGA..cm or above. However, as stated previously, the charging member having this resistivity leads to such an inconsistency that the performance of charge injection into the photosensitive member may lower and no sufficient charging is effected.
Accordingly, with regard to contact type charging assemblies or image-forming methods making use of such charging assemblies, it has been sought to solve the above problems, i.e., to achieve both of the conflicting performances one of which is to achieve good charging performance by charge injection that has not been achieved unless low-resistivity charging members are used and the other of which is to prevent the photosensitive member surface from pinhole leak which has not been prevented in low-resistivity charging members.
In the image-forming method making use of the contact charging, any faulty charging due to contamination (toner-spent) of the charging member causes faulty images, tending to cause a problem on running performance. Thus, also in the charging carried out by injecting charges into the photosensitive member, it has been a pressing need for enabling many-sheets to be printed that the influence of the faulty charging due to contamination of the charging member is prevented.
An example using the contact charging and applied to the system called cleanerless or cleaning-at-development is seen in Japanese Patent Applications Laid-Open Nos. 4-234063 and 6-230652. These publications disclose an image-forming method in which the cleaning to remove transfer residual toner from the photosensitive member is also carried out simultaneously in a back-exposure simultaneous developing system.
However, the proposals in these publications are applicable to an image-forming process in which a charge potential and a developing applied bias are formed at low electric fields. In image formation under a higher electric field charging-developing applied bias, which is conventionally applied in electrophotographic apparatus, leak may occur to cause faulty images such as lines and dots.
A method is also proposed in which the toner having adhered to the charging member is moved to the photosensitive member at the time of non-image formation so that any harmful influence from adhesion of the transfer residual toner can be prevented. However, the proposal does not mention anything about improvement in the recovery rate, in the developing step, of the toner moved to the photosensitive member, and about any effect on development that may be caused by the collection of toner in the developing step.
In addition, if the effect of cleaning the transfer residual toner is insufficient at the time of development, the subsequent toner participates in development on the photosensitive member on which the transfer residual toner is present, and hence an image formed thereat may have a higher density than its surroundings to cause positive ghost. Also, if the transfer residual toner is in a too large quantity, it may not be completely collected at the development part to cause positive memory on images. No fundamental solution of these problems has been achieved.
Light screening caused by the transfer residual toner especially causes a problem when the photosensitive member is repeatedly used on one sheet of transfer medium, i.e., when the length corresponding to one round of the photosensitive member is smaller than the length in the moving direction of the transfer medium. Since the charging, exposure and development must be performed in such a state the transfer residual toner is present on the photosensitive member, the potential at the photosensitive member surface portion where the transfer residual toner is present can not be completely dropped to make development contrast insufficient, which, in reverse development, appears on images as negative ghost, having a lower density than the surroundings. The photosensitive member having finished electrostatic transfer stands charged in a polarity reverse to the polarity of toner charge on the whole, where, because of any deterioration of charge injection performance in the photosensitive member as a result of long-term service, the transfer residual toner not controlled to have the normal charge polarity in the charging member may leak from the charging member during image formation to intercept exposure light, so that latent images are disordered and any desired potential can be attained, causing negative memory on images. It is sought to make fundamental solution of these problems.